Entity association maps are conventionally generated either manually based on existing taxonomies and encyclopedias or via a hybrid approach where pre-built taxonomies are enriched, either manually or through supervised aggregation. Examples of the former approach are Wikipedia, DMOZ's Open Directory Project or Google's Knol. These taxonomies rely on a substantial collaborative effort in order to span a significant number of topics with enough depth. Therefore, approaches of the second type have been introduced in order to either reduce the cost of building such taxonomies, or to expand their coverage.
In general, given the pace at which information is currently generated, it has become increasingly challenging to create and maintain manually created taxonomies. This is especially true on the World Wide Web, where not only the number of pages online has been increasing at a very rapid pace, but also the percentage of pages with content that quickly varies over time (e.g. news, blogs, personal pages within social networks) started assuming a predominant role. At the same time, the importance of correctly categorizing and organizing, not simply the content of such pages, but the information they contain has reached an unprecedented commercial value.